The present invention relates to a set of refractory assemblies and a method for transferring liquid metal from a first container to a second container or mold, wherein the refractories have a channel for injection of inert gas.
The present invention relates to a refractory assembly or a set of refractory assemblies for a plant for transferring liquid metal from an upstream container to a downstream container, comprising: an upstream container; a downstream container; a taphole in the upstream container; a flow regulator for regulating the flow of liquid metal through the taphole; a set of refractory assemblies which are placed between the upstream container and the downstream container in the extension of the taphole and delimit a tapping spout via which the metal flows from the upstream container into the downstream container, each refractory assembly of the tapping spout having at least one mating surface forming a joint with a corresponding surface of an adjacent refractory assembly; a shroud channel placed around the tapping spout near at least one mating surface between refractory assemblies.
Refractory assembly is understood to mean a monolithic component consisting of one or more amounts of refractory, possibly comprising other constituents, for example a metal shell. Flow regulator is understood to mean any type of device used in this technical field, such as a stopper rod, a slide gate valve, and also a simple restriction.
In a plant of this type, the presence of a regulator in the tapping spout means that, when the liquid metal is flowing, there is a pressure drop. If the tapping spout is not perfectly sealed, air can be drawn into it because of this reduced pressure. This is generally the case, in particular at the mating surfaces between the various refractory assemblies, which form the tapping spout, the sealing of which is difficult to achieve and to maintain. Air is therefore drawn in, which results in a degradation in the quality of the metal.
In order to solve this problem, it is known to create, by means of a shroud channel, an overpressure of an inert gas around the tapping spout, near each critical mating surface. Inert gas is understood to mean here a gas, which does not impair the quality of the tapped metal. Among the gases normally used may be found nobel gases, such as argon, but also gases such as nitrogen or carbon dioxide.
According to a known embodiment, a groove is formed in at least one of the mating surfaces between two adjacent refractory assemblies. This groove is fed with pressurized inert gas and thus forms an annular shroud channel placed around the tapping spout. Such an embodiment is known, for example, from U.S. 4,555,050 or EP 0, 048, 641.
In the particular case in which successive refractory assemblies are able to move with respect to each other, the use of a shroud channel is also known. French Patent Application FR 74/14636 describes a slide gate valve having two plates, each plate having a hole through which the liquid metal passes, the sliding of one plate with respect to the other enabling the flow of liquid metal to be regulated. These two plates each have, along their common mating plane, a U-shaped groove placed head to tail with respect to each other so that the arms of one of the U""s overlap the arms of the other U, and thus produce a closed shroud channel whatever the relative position of the two plates.
All these known arrangements are used to replace the induction of air by the induction of an inert gas, thereby eliminating the chemical problem associated with the liquid metal coming into contact with air.
However, these known solutions have several disadvantages.
The introduction of gas into the tapping spout is not eliminated. It is even increased because the shroud channel is at an overpressure. This is a drawback particularly in the case of transfer of metal between a tundish and a continuous-casting mold. The gas introduced into the tapping spout ends up in the mold and causes perturbations therein, such as turbulence, movement of the coverage powder and the trapping of this powder in the liquid metal. The gas entrained into the mold may furthermore become dissolved in the liquid metal and subsequently create defects in the solidified metal. These perturbations therefore degrade the quality of the metal produced.
In addition, in order to reduce the speed of the metal as it enters the mold, and thus to reduce the turbulence in this mold, many types of jet shroud tubes have an outlet cross-section greater than their inlet cross-section. The speed of flow of the liquid metal then decreases gradually. The presence of a significant quantity of gas in the tube may prevent correct operation of this type of tube: the flow may separate from the walls of the tube and the liquid metal then drops as a jet into the mold.
The quality of a mating surface between two refractory assemblies may vary while the tapping spout is being used. Defects may appear and, in particular in the case of refractory assemblies which can move with respect to each other, wear of the mating surface may lead to significant leakage.
It is therefore necessary to make the regulation of the supply of inert gas into the shroud channel more sophisticated.
One possibility is to regulate the flow of inert gas introduced into the shroud channel. In this case, if the sealing defect becomes significant, it may happen that the flow rate of inert gas is no longer high enough f or only the inert gas to enter the tapping spout. In this case, the pressure in the shroud channel becomes negative and ambient air can be drawn into the tapping spout. On the other hand, if the sealing is good, a fixed flow of inert gas is nevertheless introduced into the shroud channel, the pressure therein increases and the inert gas enters the tapping spout without this really being necessary.
Another possibility is to regulate the pressure of the inert gas as it is being introduced into the shroud channel. In this case, if the sealing defect becomes significant, the flow rate of inert gas entering the tapping spout is high, which leads to the defects mentioned above.
In practice, when the leakage rate is high it is necessary to use these two modes of regulation in alter nation, even if this means accepting a certain amount of air being drawn in rather than too great an excess of inert gas. Consequently, management of the regulation is complex and necessarily includes compromises between two types of disadvantages.
The subject of the present invention is specifically a plant for transferring liquid metal, which solves the problems explained above, and sets of refractory assemblies enabling it to be operated.
The subject of the invention is also a method of regulating the supply of inert gas into a shroud channel.
The subject of the invention is furthermore a method making it possible to improve the sealing of the mating surfaces between refractory assemblies during use of the tapping spout.
The invention relates to a set of refractory assemblies, comprising at least two refractory assemblies, which is capable of being used between an upstream container and a downstream container of a plant for transferring liquid metal, in particular steel. Such a plant generally comprises a tapping spout via which the metal flows from the upstream container into the downstream container, each refractory assembly of the tapping spout having at least one surface forming a mating surface with a corresponding surface of an adjacent refractory assembly; a flow regulator for regulating the flow of liquid metal through the tapping spout; a shroud channel placed around the tapping spout near at least one mating surface between refractory assemblies and having an inlet capable of allowing the intake of a fluid.
The said at least two refractory assemblies comprise means capable of forming the said shroud channel.
The invention is characterized in that the said shroud channel has an outlet capable of allowing a fluid to escape to the outside of the plant. In a preferred variant of the invention, the shroud channel has an inlet at one end and an outlet at the other end. Preferably, it is linear and continuous. The inlet of the shroud channel and its outlet may be provided on a single refractory assembly. The entirety of the shroud channel is then made in this refractory assembly. The shroud channel may also run through several mating surfaces of the tapping spout in succession, the continuity of the shroud channel being provided by corresponding communications of the said channel at the mating surfaces. In particular, the set of refractory assemblies may comprise two refractory assemblies, the inlet of the shroud channel being located on one of these assemblies and the outlet of the shroud channel being located on the other.
In a preferred variant of the invention, a calibrated head loss, terminated by a venting outlet, is connected to the outlet of the shroud channel. This calibrated head loss may be connected to the outlet of the shroud channel outside the set of refractory assemblies, but may also consist of a duct of small cross-section and of suitable length made within the actual refractory assembly.
The sets of refractory assemblies according to the invention may comprise plates constituting a movable slide gate valve. In this case, at least one of the plates has a first U-shaped part of the shroud channel, the arms of which U are aligned with the movement of the slide gate valve. A second plate, adjacent to the previous one, has a second U-shaped part of the shroud channel, opposite the previous one. One arm of the U of one of the plates is partially superposed an one arm of the U of the other plate for at least certain positions of the slide gate valve so as to ensure continuity of the shroud channel. The arms of the shroud channel, which are opposite the superposed arms, are offset so that there is no superposition between them, whatever the position of the slide gate valve. The parts of the shroud channel are capable of being connected together and to the adjacent refractory assemblies so as to form a continuous linear shroud channel. In the case of the plates of such a slide gate valve, the U-shaped part of the shroud channel may be placed non-symmetrically with respect to the tapping spout.
The invention also relates to a refractory assembly which can be used in a set of refractory assemblies, as described previously.
The invention furthermore relates to a plant for transferring liquid metal, in particular steel, between an upstream container and a downstream container, characterized in that it comprises a set of refractory assemblies, as described previously.
In a preferred variant, this plant comprises means capable of introducing a sealing agent into the shroud channel. The sealing agent may be a powder, and in particular a powder having particles of varying size. Included among powders which are useful as the sealing agent are graphite or other refractories, and enamels which are fusible at the temperature of the shroud channel and the viscosity of which, in the liquid state, is sufficient to close off, at least partially, the leaks in the shroud channel. The sealing agent may also be chosen from paints and resins. It may also be chosen from salts or metals.
Finally, the invention relates to a method of regulating the supply of inert gas in a plant for transferring liquid metal according to the invention. Within the scope of this method, a flow of inert gas is introduced into the shroud channel, the flow being set at a high enough value for an excess of inert gas to escape via the outlet whatever the flow rate of inert gas drawn into the tapping spout. In a preferred variant of this method, the following steps are carried out:
a flow of inert gas is injected into the shroud channel;
the pressure of the inert gas at its inlet into the shroud channel is measured;
the flow rate of inert gas injected into the shroud channel is regulated to a set value;
the flow rate of inert gas at the venting outlet is calculated;
the set value of the flow rate of inert gas injected into the shroud channel is adjusted in such a way that the flowrate of inert gas at the venting outlet is always positive.
In an improvement of this method, the flow rate of inert gas drawn into the tapping spout is determined by the difference between the flow rate of inert gas injected into the shroud channel and the flow rate of inert gas at the venting outlet, and a sealing agent is then injected into the shroud channel when the said flow rate of inert gas drawn into the tapping spout exceeds a permitted limit.
Because of the linear and continuous arrangement of the shroud channel, the circulation of the inert gas ensures that the sealing agent is transported over the entire length of this channel, thereby avoiding dead zones. The presence of the opening of the shroud channel enables any excess sealing agent to be removed to the outside of the plant.
Other features of the invention will appear on reading the description which follows, reference being made to the appended figures. In these figures: